1. Field of the Invention
The present invention relates to a recording medium, and more particularly, to a write-once-read-many type optical disk.
2. Description of the Related Art
Generally, a recording medium is classified into a semiconductor recording medium, a magnetic recording medium or an optical recording medium. As different types of optical recording media, there are a read-only type which can read data but can not write data, such as a CD-ROM (Compact Disk-Read Only Memory), an overwritable type which can write, erase and read data, and a write-once-read-many type which can write data only once.
As illustrated in FIG. 1, a conventional write-once-read-many type optical disk includes a substrate 2, a recording layer 4 of a thin amorphous TeO.sub.2 --Te--Pd layer in which data is recorded, an adhesive layer 6 and a substrate 8 formed on the adhesive layer 6. Such a write-once-read-many type optical disk is disclosed in SPIE Vol. 1499, Optical Data Storage, pp. 187-194, 1991, Shunji Ohara et al., Matsushita Electric Industrial Co., Ltd.
In operation, since the recording layer 4 is in an amorphous state before data is recorded, it has a low reflectance. If a laser beam is irradiated on the disk for recording, the recording layer 4 of the amorphous state is heated. Thereafter, the recording layer 4 gradually cools down and is crystallized. Under the crystallized state, the reflectance of the recording layer 4 is increased by 7-9%. This means that data is recorded. That is, since there is a difference between the reflectances prior to and after irradiation of the laser beam on the disk, data can be accurately read.
However, the recording layer 4 which is the thin amorphous TeO.sub.2 --Te--Pd layer has disadvantages as described below. First, during recording, a laser recording power is high (12-13 mW). Therefore, in order to raise a recording density, a shortwave laser should be used. However, the power of the shortwave laser is very low industrially. Second, the difference in the reflectance between an initial non-data recording state and an after-data recording state is low. Third, since a melting point of the thin amorphous TeO.sub.2 --Te--Pd layer is low (about 600.degree. C.), the amorphous TeO.sub.2 --Te--Pd layer shows a growing tendency toward crystallization with the lapse of use over time and thus the storage lifetime of the recorded data is short. Fourth, elements Te and Pd used for the recording layer 4 are expensive. Fifth, the element Te is bad for the human body.